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Example: Control-Flow Nesting

Let’s turn to an example to make this nesting concept more concrete. The following module file, nestexc.py, defines two functions. action2 is coded to trigger an exception (you can’t add numbers and sequences), and action1 wraps a call to action2 in a try handler, to catch the exception:

def action2():
    print(1 + [])            # Generate TypeError

def action1():
    try:
        action2()
    except TypeError:        # Most recent matching try
        print('inner try')

try:
    action1()
except TypeError:            # Here, only if action1 re-raises
    print('outer try')

% python nestexc.py
inner try

Notice, though, that the top-level module code at the bottom of the file wraps a call to action1 in a try handler, too. When action2 triggers the TypeError exception, there will be two active try statements—the one in action1, and the one at the top level of the module file. Python picks and runs just the most recent try with a matching except, which in this case is the try inside action1.

As I’ve mentioned, the place where an exception winds up jumping to depends on the control flow through the program at runtime. Because of this, to know where you will go, you need to know where you’ve been. In this case, where exceptions are handled is more a function of control flow than of statement syntax. However, we can also nest exception handlers syntactically—an equivalent case we’ll look at next.